As taught in British Pat. No. 890,432, bisphenols are readily decomposed when heated, and alkalis readily catalyze bisphenol decomposition. Prior to the present invention, therefore, the production of substantially anhydrous alkali metal diphenoxide salts of the formula, ##STR1## involving direct contact between alkali metal ion and bisphenol often required carefully controlled and multi-step procedures. As used hereinafter, the term "substantially anhydrous" signifies the bisphenol salt has less than 1% by weight of water based on the weight of bisphenol salt as determined with Karl Fischer reagent, where M is an alkali metal ion and Z is a divalent organic radical defined more particularly below. For example, White, U.S. Pat. No. 3,852,242, is based on the initial formation of an alkali metal alkoxide which is thereafter reacted with a bisphenol in the presence of a hydrocarbon solvent. The excess hydrocarbon is stripped from the mixture and the resulting bisphenol salt dried under reduced pressure at 100.degree. C. for one hour.
The present invention is based on the discovery that bisphenols neutralized with a substantially stoichiometric equivalent of an alkali metal hydroxide are very stable at elevated temperatures and that substantially anhydrous diphenoxide salts of formula (1) can be made more directly and faster and recovered as a substantially pure and anhydrous powder by heating at temperatures up to 350.degree. C., a hydrate of the diphenoxide salt, such as disodium bisphenol-A hexahydrate, or an aqueous mixture of bisphenol and alkali metal hydroxide, for example, bisphenol-A and sodium hydroxide, where the bisphenol and the alkali metal hydroxide are present in the aqueous mixture in substantially stoichiometric proportions.